Methods have been disclosed in the prior art to increase the fuel injection pressure in such a way for improving the performance of internal combustion engines having a cylinder injection system and for improving the reduction of exhaust gas that the fuel is atomized into small droplets. The fuel supply system of internal combustion engines is therefore designed so as to achieve the typical values for current systems for a high pressure of 4 to 10 Mpa.
Fuel supply systems disclosed in the prior art, such as the one described, for example, in DE 41 26 640 A1, are divided into a low-pressure system and a high-pressure system. The fuel, which is pre-supplied from the fuel tank by means of a low-pressure fuel pump and is under a low preliminary pressure, is delivered to the high-pressure pump, which is designed as a radial piston pump. The fuel pressure is raised further to a predetermined pressure value. The system pressure is regulated in the high-pressure system, wherein the actual pressure is detected using a high-pressure sensor, and compared with a desired pressure in an engine control unit, and a control value for a pressure-limiting valve is determined. The required pressure is adjusted in a high-pressure common rail and the excess fuel quantity is throttled off by way of a return flow line to the tank. The high pressure is regulated to the high-pressure desired value independently of the fuel quantity injected into the internal combustion engine. The excess fuel quantity can also be guided in a targeted manner in an additional rinse flow. However, this poses the problem of the fuel getting heated excessively.
According to DE 196 52 831 A1, instead of being led back to the tank, the fuel can also be guided back to the high-pressure pump and again compressed there immediately, thereby improving the efficiency of the fuel supply system.
Likewise, the desired pressure of the low-pressure system is usually regulated and specified variably depending on the vapor pressure curve of the worst-case fuel and the adaptation values determined.
The actual pressure detected using a low-pressure sensor is compared to the desired pressure and processed in an engine control unit to form a regulator response, wherein simultaneously an adapted adaptation value for the desired pressure is determined and adjusted. A characteristic map having values for the required delivery capacity of the low-pressure pump, which is usually designed as an electric fuel pump, is addressed in the engine control unit by means of the regulator response of the low-pressure regulation, desired pressure, adaptation value and the current fuel mass flow, and a value for the delivery capacity of the pump is determined and outputted.
The desired pressure usually assumes its highest values during hot start and cold start. A formation of vapor bubbles must be prevented during hot start since the high-pressure pump can no longer generate high pressure upon the formation of vapor bubbles. During cold start, the injection valves have to inject a large quantity of fuel into the combustion chamber when the high-pressure pump is not yet active.
However, the delivery capacity of the low-pressure pump reduces with the increasing preliminary pressure, so that at definite operating points and at a high desired pressure, the low-pressure pump is loaded excessively and is pushed to its delivery limits under certain circumstances.
In order to prevent the formation of vapor bubbles in the internal combustion engine on the one hand, while on the other hand ensuring the supply of fuel to the internal combustion in all operating states, DE 199 51 410 A1 suggests the adjustment of the lowest possible preliminary pressure that would reliably avoid any fuel vaporization. For this purpose, the current temperature of the fuel in the high-pressure pump is determined, and the low-pressure pump is controlled or regulated in such a way depending on the determined temperature that the low-pressure pump generates the determined preliminary pressure.
However, in addition to the temperature, the quality of the fuel also has a decisive influence on the formation of vapor bubbles since different fuels vaporize at different temperatures. In order to ensure a secure mode of operation of the internal combustion engine, the preliminary pressure is usually adjusted for the worst-case scenario with a large tolerance control. An optimum adjustment of the preliminary pressure is thus possible, if at all, using additional measures, such as for example, an additional recognition of a refueling operation. Furthermore, those system characteristics of the pressure systems that either cannot be compensated using the known pressure regulations or can be compensated only using further increased tolerances, change during the service life of the internal combustion engine. This likewise leads to an increased pressure level in the fuel supply system and thus to an unnecessarily high power consumption of the low-pressure pump.